As is well known, metallic conductor transmission media have been used widely in communications. Such media typically include a plurality of twisted pairs of insulated conductors which comprise a core. Each insulated conductor typically includes a metallic conductor having a layer of an insulation material thereabout. The core typically is enclosed in a sheath system which includes at least a plastic jacket.
Although over the last decade, optical fiber transmission has enjoyed a spectacular climb in use, metallic conductors continue to be used. However, in such a competitive environment, it behooves any manufacturer of cables which include insulated metallic conductors, to overcome any problems which have manifested themselves.
One such problem relates to an insulation system which is used to enclose each metallic conductor. Typically, that insulation system comprises an inner layer of a cellular or expanded insulation whereas an outer layer comprises a solid insulation material. In many instances, the insulation material is a composition which comprises a polyolefin plastic material, and, more particularly, a polyethylene plastic material and a stabilization system.
Such insulation material has been found to possess excellent mechanical and electrical properties. However, it also has been determined that the relatively low thermal stability of polyolefins may lead to a problem after long term use. Unless this problem is addressed, the insulation material may crack where exposed to relatively high temperatures. Such temperatures may occur, for example, in areas of the southwestern portions of the United States. The cracking of conductor insulation occurs when portions of insulated conductors of aerial or buried cables become exposed to air in splicing environments such as in closures, for example.
There is some thought that the lack of thermal stability may be caused by the extraction of constituents of a stabilization system of the insulation composition by filling materials which are used widely in communications cables. Further, it has been shown that an adverse reaction occurs between the surface of a copper conductor and the stabilization system of the insulation material. As a result, the copper of the metallic conductor catalyzes the oxidation of the polyethylene insulation which then deteriorates at an accelerated rate. Copper catalyzed oxidation of polyolefin insulation leads to the premature failure of communications cables.
The stabilization of cellular insulation over copper conductors has been discussed in an article authored by M. G. Chan, V. J. Kuck, F. C. Schilling, K. D. Dye and L. D. Loan entitled "Stabilization of Foamed Polyethylene Communication Cable Over Copper Conductors" which appeared in the proceedings of the Thirteenth Annual International Conference on Advances In The Stabilization and Degradation of Polymers held in Luzern, Switzerland on May 22-24, 1991.
Manufacturers have addressed the problem of stabilization, and, as a solution, have included in the composition of the insulation material an antioxidant and a metal deactivator. See, U.S. Pat. No. 3,668,298 which issued on Jun. 6, 1972 in the name of W. L. Hawkins. Further, more recently, the levels of antioxidant and of metal deactivator constituents in the insulation composition have been increased. However, it was believed that there were certain outer limits of the amount of stabilizer the should be used. For example, it was believed that the addition of stabilizer including antioxidant and metal deactivator functions at a level of about 0.25% by weight would satisfy all the requirements for long term use.
What is sought after and what appears not to be available in the prior art is a cable which includes a conductor insulated with a polyolefin composition which has sufficient thermal stability to cause the integrity of metallic conductor insulation to be maintained over a relatively long period of time as predicted by currently used tests. The sought-after composition desirably should be reasonable in cost and easily applied to a metallic conductor without the need of additional capital investment.